Transforming growth factor-β (TGF-β) is secreated in an inactive form, and when activated, forms a heter-tetrameric receptor complex comprised of type 1 (TGFBR1) and type 2 (TGFBR2) receptors. The TGFBR2 gene provides instructions for making a protein called transforming growth factor-beta receptor type 2. This receptor transmits signals from the cell surface into the cell through a process called signal transduction.
To carry out its signaling function, the TGF-β receptor type 2 (TGFBR2 or TGFβR2) spans the cell membrane, so that one end of the protein projects from the outer surface of the cell (the extracellular domain) and the other end remains inside the cell (the intracellular domain). TGF-β binds to the extracellular domain of TGFBR2, which activates this receptor and allows it to bind to another receptor, TGFBR1, on the cell surface. These three proteins form a complex, triggering signal transduction by activating other proteins in a signaling pathway called the TGF-β pathway.
Signals transmitted by the TGF-β receptor complex triggers various responses by the cell, including cellular differentiation, cellular proliferation, cellular motility, and cell apoptosis. Overexpression of TGFBR2 can, therefore, be disruptive to cells.
Secondary mutations in the tyrosine kinase domain (KD) is one of the most common causes of acquired clinical resistance to small molecule tyrosine kinase inhibitors (TKIs) in human cancer. Recent pharmaceutical efforts have focused on the development of “type II” kinase inhibitors, which bind to a relatively non-conserved inactive kinase conformation and exploit an allosteric site adjacent to the ATP-binding pocket as a potential means to increase kinase selectivity. Mutations in FLT3 are the common genetic alteration in patients with acute myeloid leukemia (AML) (TCGA, N Engl J Med. 2013, 368: 2059-74) and are primarily comprised of constitutively activating internal tandem duplication (ITD) mutations (of 1-100 amino acids) in the juxtamembrane domain, and to a lesser extent, point mutations, typically within the kinase activation loop. Secondary KD mutations in FLT3-ITD that can cause resistance to the highly potent type II FLT3 inhibitors, such as, quizartinib, which achieved a composite complete remission (CRc) rate of about 50% in relapsed or chemotherapy-refractory FLT3-ITD+ AML patients treated in large phase II monotherapy studies (Tallman et al., Blood, 2013; 122:494). An in vitro saturation mutagenesis screen of FLT3-ITD identified five quizartinib-resistant KD mutations at three residues: the “gatekeeper” F691 residue, and two amino acid positions within the kinase activation loop (D835 and Y842), a surprisingly limited spectrum of mutations for a type II inhibitor. Mutation at residue D835V/Y/F was subsequently identified in each of eight samples analyzed at the time of acquired clinical resistance to quizartinib (Smith et al., Nature, 2012; 485:260-3). This finding validated FLT3 as a therapeutic target in AML. The type II multikinase inhibitor sorafenib, which also has some clinical activity in FLT3-ITD+ AML, is ineffective against all identified quizartinib resistance-causing mutants, in addition to other mutant isoforms (Smith et al.). The type I inhibitor crenolanib has been identified a type I inhibitor of quizartinib-resistant D835 mutants (Zimmerman et al. Blood, 2013; 122:3607-15).
Accordingly, there is a need for novel compounds that inhibit one or both TGFBR2 and FLT3 kinases with secondary mutations for the treatment of various diseases.